1. Field of the Invention
The invention relates generally to the field of seismic data acquisition and processing to image subsurface Earth formation structures. More specifically, the invention relates to methods for acquiring and processing passive seismic data, that is, seismic data acquired without the use of a controllable source.
2. Background Art
Passive seismic emission tomography is a process in which an array of seismic sensors is deployed in a selected pattern on the Earth's surface (or on the water bottom in marine surveys) and seismic energy is detected at the sensors that emanates from various seismic events occurring within the Earth's subsurface. Processing the signals detected by the sensors is used to determine, among other things, the position in the Earth's subsurface at which the various seismic events took place.
Applications for passive seismic emission tomography include, for example, determining the point of origin of microearthquakes caused by movement along geologic faults (breaks in rock layers or formations), movement of fluid in subsurface reservoirs, and monitoring of movement of proppant-filled fluid injected into subsurface reservoirs to increase the effective wellbore radius of wellbores drilled through hydrocarbon-producing subsurface Earth formations (“fracturing”). The latter application, known as “frac monitoring” is intended to enable the wellbore operator to determine, with respect to time, the direction and velocity at which the proppant filled fluid moves through particular subsurface Earth formations.
One particular type of passive seismic emission tomography that has been useful is described in U.S. Patent Application Publication No. 2008/0068929 filed by Duncan et al., the underlying patent application for which is assigned to the assignee of the present invention. A method disclosed in the '929 publication includes detecting seismic signals from within the Earth's subsurface over a time period using an array of seismic sensors. The seismic signals are generated by seismic events within the Earth's subsurface. The method includes transforming seismic signals recorded at selected positions into a domain of possible spatial positions of a source of seismic events. An origin in spatial position and time of at least one seismic event is determined from space and time distribution of at least one attribute of the transformed seismic data. A limitation of the method disclosed in the '929 publication relates to the spatial orientation and radiation pattern of the event giving rise to seismic signals detected at the Earth's surface. Depending on the orientation of the source event, its radiation pattern, and the geometry of the sensor array at the surface, some of the sensors in the array may detect a rarefaction as the initial portion of a detected seismic event. Other sensors in the array may detect a compression as the initial portion of the same event. In such cases, if the polarity of the detected seismic events is not correctly identified with reference to the orientation of the seismic source event, the results may be ambiguous.
Polarization correction techniques for microearthquakes are known in the art. For example, Charles A. Langston, SOURCE INVERSION OF SEISMIC WAVEFORMS: THE KOYNA, INDIA, EARTHQUAKES OF 13 Sep. 1967, Bulletin of the Seismological Society of America, Vol. 71, No. 1 (1981) describes various techniques for identifying the orientation of the source mechanism of earthquakes. Generally, such techniques may be described as follows. A hypocenter of a seismic event in the subsurface is determined from passive seismic data recorded at the surface or in wellbores. A possible mechanism (orientation and displacement) which would result in the measured seismic event is selected, given the located hypocenter. Seismic waveforms (signals) are synthesized for each seismic receiver for the selected event. Such synthesis uses as input the seismic properties of the formations existing between the hypocenter and each seismic sensor. The measured seismic signals from each seismic sensor are compared with the synthesized seismic signals. A different source mechanism is selected, and the synthesis is repeated. The source mechanism that results in the closest match between the synthetic seismic signals and the measured seismic signals is selected as the most likely source mechanism.
The foregoing mechanism determination is impracticable for use in passive seismic techniques such as those used for fluid front monitoring, e.g., those described in the '929 publication. Reasons for such impracticability include that in fracture monitoring, for example, there are multiple seismic events closely spaced in time and position. Further, there are typically large numbers of seismic sensors disposed at the surface for which synthesis would have to be performed.
There exists a need for source event polarization correction of seismic signals for improved seismic event source location determination.